Abstract: There is provided a system for storing and injecting an additive into exhaust gas from an internal combustion engine, the system comprising a tank for storing the additive, an injector, and a pump for driving the additive from the tank to the injector via an injection channel, the system also comprising a purge device mounted between the pump and the injection channel. The purge device comprises a chamber and piston moving equipment slidably mounted in the chamber, the purge device being designed so that: movement of the moving equipment in one of its sliding directions causes a passage to open into the chamber for passing the additive to the injection channel; and movement of the moving equipment in its other sliding direction creates suction within the chamber, which causes at least some of the additive contained in the injection channel to return into the chamber.